1. Field of the Invention
The present invention relates to a power supply apparatus using chargeable batteries. Portable devices such as a notebook personal computer, etc. can usually be operated by both AC mains and a battery. When the portable devices are operated by the AC mains, an AC adapter is used. The AC adaptor is designed so as to charge the batteries while also supplying a load with power. The present invention relates to a power supply apparatus for such a device installed with a plurality of chargeable batteries.
2. Description of the Related Art
FIG. 1 shows the configuration of a conventional power supply apparatus with chargeable batteries. The operation of this conventional power supply apparatus is described in detail in the following prior application.
Laid-open Patent Publication No. 8-137814 (9-322431)
Inventor: Seiya Kitagawa
Title of the Invention: Power supply apparatus
In FIG. 1, an AC adaptor is connected to the external power source terminal, power supplied from the AC adaptor drives a load 10 through a diode 9, and also charges a battery 14 through a DC--DC converter 11 for charging. When the AC adaptor is not connected or the voltage drops to an abnormally low level, the potential of the cathode side of the diode 9 falls, this fall of potential is detected by a charge/discharge monitor circuit 16, and the PWM (Pulse With Modulation) control circuit 25 inside the DC--DC converter 11 is controlled by the amount of potential drop. Thus, a FET 21 is always kept on, and the discharge current of the battery 14 is supplied to the load 10 through the DC--DC converter 11.
The DC--DC converter 11 is mainly used to regulate the voltage between the AC adaptor and the battery 14. The DC--DC converter 11 controls the charge to the battery 14 by switching on/off the FET 21 according to the control of a voltage error amplifier, a current error amplifier and a PWM comparator inside the PWM control circuit 25. For the details of these operations, see the above-mentioned prior application.
In the conventional example shown in FIG. 1, since only one chargeable battery 14 (a single package) can be used, there was a problem that the operation hours of a device driven by the battery cannot be extended by connecting a plurality of chargeable batteries in parallel. This is because when there is an imbalance in the charge states of batteries connected in parallel, energy flows from charged batteries to less-charged batteries, and such a charging overcurrent which occurs in this situation may damage the batteries.